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Original article
09 2024
:17;
105874
doi:
10.1016/j.arabjc.2024.105874

An insight on the role of Pr in the photocatalytic efficiency of mesoporous CeO2@C

, , , , ,
a Xingzhi College, College of Chemistry and Materials Science, Zhejiang Normal University, 321004 Jinhua, PR China
b Department of Chemical, Materials and Production Engineering, University Federico II, P.le V. Tecchio 80, 80125 Naples, Italy

⁎Corresponding author. sky54@zjnu.cn (Shiyou Hao)

Received: , Accepted: ,
© The Author(s) 2024
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
1 Fanfan Zhang, Chenxin Mao and Yanhong Tu, equally contributed to this paper, are the co-first authors.

Abstract

Using Ce(NO3)3·6H2O and Pr(NO3)3·6H2O as raw materials, ammonium hydroxide as precipitate, acid red 14 (AR14) as carbon source, Ce-Pr-O@C was synthesized in this study. The structural properties of the resulted samples were characterized by XRD, N2 adsorption–desorption, Raman, TEM, UV–vis, FT-IR, and XPS techniques. The photocatalytic efficiency of Ce-Pr-O@C was evaluated by the degradation of AR14. Effect of Pr doping on the photocatalytic efficiency of Ce-Pr-O@C was focused on. The results show that 25% Pr doping amount is benefit for the formation of mesopores, improvement of hydroxyl group content, enhancement of bonding rate, resulting in the promotion of adsorption capacity of organic pollutants and separation of photogenerated electron and hole and thus increasing of photocatatlytic efficiency. Moreover, effect of carbon loading amount on the photocatatlytic efficiency was also investigated.

Keywords

Pr doping
Ce-Pr-O@C
Synthesis
Photocatalysis
AR14
1

1 Introduction

As we know, environmental pollution resulting from the organic-based pollutants is more and more serious with the rapid development of economy. At present, a number of technologies, such as physical, chemical, biological, and thermal methods, are used to remove these organic pollutants (Dai et al., 2020; Nidheesh et al., 2018; Kanaujiya et al., 2019; Lu et al., 2018). Among the proposed technologies, photocatalysis received great attention because pollutants can be broken down into innocuous small molecules such as CO2 and H2O by semiconductors under the irradiation of sunlight or visible light or ultraviolet light (Guo et al., 2023; Cui et al., 2023; Pavel et al., 2023). It is well known that the main factors affecting photocatalytic efficiency are as follows: (1) pollutant adsorption capacity, (2) light absorption intensity, (3) separation efficiency of the photogenerated electrons and holes. Consequently, a number of strategies have been applied to improve the photocatalytic efficiency based on these three aspects, such as surface engineering technology, doping, defects engineering and so on (Pan et al., 2017; Franceschi et al., 2020; Xiong et al., 2018). Due to high adsorption capacity for organic pollutants, strong visible light absorption, and efficient electronic transmission performance, carbon materials such as graphene, C3N4, activated carbon, etc. were often chosen as good cocatalysts to improve photocatalytic efficiency (Mishra et al., 2023; Wudil et al., 2023; Xu et al., 2022).

As an alternative photocatalyst for the degradation of organic contaminants, CeO2 has received much attention because of its potential visible light absorption (Chen et al., 2012), and easily achievable state from Ce (IV) − Ce (III) in the redox cycle (Verma and Samdarshi, 2016). However, the application of CeO2 in the degradation of organic contaminants is limited due to its large band gap (3.2 eV) and the fast recombination between the photogenerated electrons and holes (Saravanakumar et al., 2016; Liyanage et al., 2014). Inspired by the report of carbon-based photocatalysts, a series of CeO2@C composite materials were prepared and used as photocatalysts. Among all the carbon sources, grapheme (Du et al., 2020), C3N4 (Li et al., 2016), and activated carbon (Jayakumar et al., 2019) are often selected. Furthermore, other materials are also used as carbon source to synthesize CeO2@C photocatalyst (Ye et al., 2019; Qian et al., 2018). Although the photocatalytic efficiency of the synthesized CeO2@C materials was enhanced compared with pure CeO2, the cost and the complex transformation process of carbon source always hinder the practical application of CeO2@C. Consequently, it is imperative to find other economic and effective methods to design CeO2@C materials. It is well known that dyes are often used as target organic pollutants to estimate the photocatalytic efficiency (Matussin et al., 2023; Kumari et al., 2023). Most of the selected dyes are rich in carbon element, which may be a good precursor of carbon source. Nevertheless, the relevant work changing dyes into carbon source was ignored. How can we carry out this work? Recently, we found that acid dyes such as acid red 14 and acid orange 7 can be efficiently adsorbed by CeO2·xH2O to form CeO2·xH2O@dyes (Wang et al., 2019). Using CeO2·xH2O@dyes as precursor, a novel CeO2@C material was obtained. The results of photocatalysis showed that the photocatalytic efficiency of the resulted material was higher than that of CeO2@activated carbon due to more efficient carbon bonding rate in the prepared CeO2@C, resulting in a better separation efficiency of photogenerated electrons and holes (Wang et al., 2020).

As mentioned above, doping is an effective way to enhance photocatalytic efficiency. In order to further improve the photocatalytic efficiency of CeO2@C, other components doped CeO2@C materials were always employed by many researchers (Rashid et al., 2019; Jiang et al., 2019; Chu et al., 2011). However, almost of these ternary photocatalysts were obtained by the physical mixing of CeO2, C and corresponding components, which greatly hinder the formation of carbon bonds, resulting in a lower separation efficiency of photogenerated electrons and holes. Furthermore, the selected components are not easy to dope in CeO2 because of structural difference. How can we improve the doping efficiency and carbon bonding rate in CeO2@C?

Because of the same structure (fluorite-like) of CeO2 and Pr6O11 and similar ion radius of Pr4+ (0.096 nm) and Ce4+ (0.097 nm), Pr can be easily doped in CeO2 to form solid solution (Zhang et al., 2018; Fan et al., 2017; Cruz Pacheco et al., 2017). As reported in the previous literatures, oxygen vacancies were created in CeO2 and thus the Lewis acidity of CeO2 was enhanced after Pr doping (Zhang et al., 2016; Paunovi et al., 2012). Due to the positive charge of oxygen vacancies, the photogenerated electrons are easy to be captured by these positive sites and hence the photocatlytic efficiency of Pr doped CeO2 was greatly improved (Hao et al., 2014a,b). It can be inferred from the above discussion that Pr is indeed easy to be doped in CeO2, but what will happen when Pr is doped into CeO2@C?

Driven by this issue, we designed and synthesized Pr doped CeO2@C obtained from CeO2·xH2O@dyes and focused on the effect of Pr doping amount on the carbon bonding rate and then the separation efficiency of photogenerated electrons and holes. The results show that it is helpful for the improvement of carbon bonding rate when Pr doping amount is relatively high. Moreover, the structure and property of CeO2@C were also investigated after doping of Pr.

2

2 Experimental

2.1

2.1 Chemicals

Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O), praseodymium (III) nitrate hexahydrate (Pr(NO3)3·6H2O), absolute ethanol, sodium hydroxide (NaOH), ammonium hydroxide (NH3·H2O, 27 %), 37 % fuming hydrochloric acid, 1, 4-benzoquinone, terephthalic acid, and KI were purchased from Sinopharm Chemical Reagent Co. Acid Red 14 (AR14, analytical grade) was purchased from Nanjing Chemical Reagent Co., Ltd. All the chemical reagents were used without further purification. Deionized water was obtained from Millipore Milli-Q® ultrapure water purification systems.

2.2

2.2 Synthesis

Typically, 1.0 g Ce(NO3)3·6H2O and a given mass of Pr(NO3)3·6H2O were dissolved in 100 mL of deionized water at room temperature under stirring (the mass ratios of Pr(NO3)3·6H2O to Ce(NO3)3·6H2O are 0, 5 %, 15 %, 25 %, 35 %, respectively). After attaining the complete dissolution of the salts, the pH of the solution was set to 8.5 by adding dropwise a proper amount of ammonium hydroxide (NH3·H2O, 27 %). After continuous stirring for 30 min, the solution was aged in a 25 °C water bath for 3 h. The product was filtered, and the solid was washed 3 times with deionized water and absolute ethanol. The washed solid was then dried at 60 °C overnight and a pale-yellow precipitate (Ce-Pr-OH) was obtained, which was used to adsorb AR14 from water solutions with different concentrations (varying from 0.05 to 0.1 mM). The adsorption process was carried out as follows: 0.1 g of Ce-Pr-OH and 100 mL of AR14 solution were added in a 200 mL beaker and stirred vigorously for 30 min at room temperature. Afterward, the supernatant was filtered and Ce-Pr-OH@AR14 was obtained. Ce-Pr-O@C was then obtained by calcination of Ce-Pr-OH@AR14 at 600 °C for 3 h with a heating ramp of 5 °C/min in N2 atmosphere. Based on the initial mass ratios of Pr(NO3)3·6H2O to Ce(NO3)3·6H2O, the resulted samples are thereafter denoted as CeO2@C, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C. Unless otherwise specified, the carbon in the samples resulted from the adsorption of 0.075 mol/L AR14 solution.

2.3

2.3 Characterization

X-ray diffraction (XRD) patterns were collected on a Philips PW3040/60 powder diffractometer using CuKα radiation (λ = 0.154 nm). N2 adsorption isotherms were measured with a Micromeritics ASAP 2020 apparatus at −196 °C and the specific surface area of the investigated samples was calculated using the multi-point Brunauer-Emmett-Teller (BET) method. TEM images were acquired by a 2100 JEOL working at 200 kV. The diffuse reflectance spectra of the samples over a range of 200–800 nm were recorded by a Nicolet Evolution 500 Scan UV–vis system. FT-IR spectra were recorded by a Nicolet Nexus 670 spectrometer with a resolution of 4 cm−1 using the KBr pellet method. Raman scattering analysis was performed on a Renishaw RM1000 Raman spectrometer with a 514 nm excitation laser light. The photoluminescence (PL) spectra of the samples were obtained at room temperature by a spectrofluorometer (NanoLOG-TCSPC, Horiba Jobin Yvon, USA) with an excitation wavelength of 325 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a RBO upgraded PHI-5000C ESCA system (Perkin Elmer) using monochromated Al Kα X-ray radiation (E = 1486.6 eV) at 250 W. All binding energies were calibrated using carbon (C1S = 284.6 eV) as a reference.

2.4

2.4 Photocatalytic tests

The photocatalytic activity of Ce-Pr-O@C or CeO2 was evaluated by carrying out AR14 photodegradation tests under visible light irradiation. In a typical experiment, 20 mg of photocatalyst was dispersed under magnetic stirring into 20 mL of AR14 solution at different concentrations (ranging from 0.075 to 0.15 mM). Afterwards, the suspensions were stirred in the dark for 30 min to reach the equilibrium. At given time intervals, a small amount of suspension was withdrawn and centrifuged to remove the photocatalyst. The residual AR14 levels in the filtrates were then analyzed by recording the variations of the absorbance at 515 nm with a UV–vis spectrophotometer (Evolution 500LC). The removal efficiency ŋ of AR14 was evaluated according to the following equation:

(1)
ŋ = (A0 − A)/A0 × 100 % where A0 is the initial absorbance of AR14 and A is the absorbance of AR14 in the filtrates.

3

3 Results and discussion

3.1

3.1 Structural properties

The powder X-ray diffraction (XRD) patterns of CeO2, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C are shown in Fig. 1a. Crystalline phases were identified in comparison with the cubic CeO2 ICDD file (PDF no. 81–0792). The results of Fig. 1a illustrate that the structure of all Ce-Pr-O@C and CeO2@C samples is similar to that of pure CeO2 (fluorite-like), and the diffraction peaks of Pr oxides are not detected. The possible reason may be that the Ce-Pr-O solid solution was formed due to the similar structure of CeO2 and Pr oxides (Zhang et al., 2018; Fan et al., 2017; Cruz Pacheco et al., 2017). The adsorption–desorption isotherms of N2 on CeO2@C and Ce-Pr-O@C compounds at −196 °C are shown in Fig. 1b. It can be found that a type IV isotherm with a hysteresis loop between type H3 and H4 is formed for all isotherms (Ji et al., 2008). From Fig. 1b, it can be concluded that mesoporous structure has been formed in all samples due to a wide range of relative pressure (P/P0 = 0.3–0.99) for the N2 capillary condensation. It is interesting to find that the range of relative pressure for the N2 capillary condensation over Ce-Pr-O@C (for example, P/P0 = 0.4–0.99 over 25 %Ce-Pr-O@C) is wider than that of CeO2@C (P/P0 = 0.3–0.85). The fact may be that introducing Pr into CeO2 is benefit for the decomposition of AR14 and thus the formation of mesoporous pores. The pore size of CeO2@C, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C is 3.6, 5.2, 5.6, 4.8, 4.7 nm, respectively, and the corresponding BET surface area is 76, 82, 84, 102, 98 m2/g. It can be seen from the above data that the pore size and BET surface area of 25 %Ce-Pr-O@C are higher than those of other samples. The most possible reason may be that the content of Pr in 35 %Ce-Pr-O@C decrease, which can be proved by the results of XPS spectra of Pr 3d, resulting in a decrease of the decomposition of AR14 and then smaller pore size and BET surface area for the 35 %Ce-Pr-O@C.

XRD patterns of CeO2, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C (a) and N2 adsorption–desorption isotherms of CeO2@C, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C at −196 °C (b).
Fig. 1
XRD patterns of CeO2, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C (a) and N2 adsorption–desorption isotherms of CeO2@C, 5 %Ce-Pr-O@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C at −196 °C (b).

The Raman spectra in the range of 1200–2100 cm−1 of Ce-Pr-O@C and CeO2 are shown in Fig. 2a. For CeO2, no characteristic peak is observed within the detected range. However, two obvious peaks at around 1350 and 1575 cm−1 can be found over CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C, which is related to the D- and G-bands, respectively (Xue et al., 2015). According to the corresponding reports (Sim et al., 2014; Chen, et al., 2010), the D-band can be ascribed to the hexagonal graphitic structure and the amorphous carbon, and the G-band can be associated with the characteristic sp2-bonded carbon of E2g mode of graphite. Therefore, it can be inferred from Fig. 2a that carbon is produced and loaded on the surface of the resulted samples. This is further proved by the results of Fig. S1. The optical properties of CeO2, CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C were measured using UV–vis absorbance and the results are shown in Fig. 2b. From Fig. 2b, it is obvious that the photocatalysts encapsulated carbon layer display a strong capability of visible light absorption compared with CeO2. The visible light absorption intensity of CeO2@C increase with increasing of Pr content in the as-prepared samples, which is related to the oxygen vacancy concentration (Paunovi et al., 2012). From the results of XPS spectra of Pr 3d (Fig. 7b), it can be found that Pr3+ was produced in the resulted samples, resulting in the formation of oxygen vacancy due to the replacement of Ce4+ by Pr3+. The oxygen vacancy concentration increase with increasing of Pr content in the as-prepared samples, which can lead to a better absorption of visible light because oxygen vacancy can reduce the band gap of CeO2 (Hao et al., 2014a,b).

Raman spectra (a) and diffuse reflectance UV–vis spectra (b) of CeO2, CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C.
Fig. 2
Raman spectra (a) and diffuse reflectance UV–vis spectra (b) of CeO2, CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C.

3.2

3.2 Photocatalytic activity

From the results of Fig. 4, it can be seen that the photocatalytic efficiency of 25 %Ce-Pr-O@C is higher than that of other Ce-Pr-O@C samples. Therefore, 25 %Ce-Pr-O@C was chosen as the representative to compare its photocatalytic efficiency of AR14 with that of CeO2 and CeO2@C, and the results are shown in Fig. 3a. From Fig. 3a, it is clear that introducing carbon is helpful for the improvement of photocatalytic efficiency of CeO2, and that the degradation efficiency of AR14 is further enhanced after doping Pr in CeO2@C. In order to find the internal reason for this phenomenon, photoluminescence (PL) technique was used to investigate electron-hole recombination and the results are presented in Fig. 3b. From Fig. 3b, it is easy to find that the recombination of electron-hole over 25 %Ce-Pr-O@C is much lower than that of CeO2@C and CeO2 and that the carbon layer on the surface of CeO2 is benefit for the retardation of electron-hole. As mentioned above, carbon material is an excellent conductor of electrons, which will improve the separation efficiency of electron-hole. On the other hand, doping Pr in CeO2 can enhance the concentration of oxygen vacancy and photogenerated holes can be easily captured by these oxygen vacancies, resulting in efficient separation of electron-hole (Hao et al., 2014a,b). Consequently, the photocatalytic efficiency of 25 %Ce-Pr-O@C is much higher than those of CeO2@C and CeO2. From the results of Fig. 2S, comparing FT-IR analysis carried out on 25 %Ce-Pr-O@C as synthesized, after AR14 adsorption and after irradiation, it can be inferred that the removal of AR14 was due to photodegradation (See Supporting Information for further details).

UV–vis spectra of 0.1 mM AR14 after photodegradation over different photocatalysts under visible light irradiation for 120 min (pH = 5, m photocatalyst = 20 mg, VAR14 = 20 mL) (a) and PL spectra of CeO2, CeO2@C, and 25 %Ce-Pr-O@C (b).
Fig. 3
UV–vis spectra of 0.1 mM AR14 after photodegradation over different photocatalysts under visible light irradiation for 120 min (pH = 5, m photocatalyst = 20 mg, VAR14 = 20 mL) (a) and PL spectra of CeO2, CeO2@C, and 25 %Ce-Pr-O@C (b).

As reported previous, Pr doping content can play an important role in the concentration of oxygen vacancy (Paunovi et al., 2012), which will greatly affect the photocatalytic efficiency. Effect of Pr doping content on the degradation of AR14 is shown in Fig. 4, and the results illustrate that the degradation efficiency of Ce-Pr-O@C is lower than that of CeO2@C when Pr doping content is less than 15 % and that the removal of AR14 increase when Pr doping content exceed 15 %. It can be seen from Fig. 4 that the optimum Pr doping content is 25 %. Why does this happen?

Effect of Pr doping content on the degradation of AR14 (pH = 5, mphotocatalyst = 20 mg, VAR14 = 20 mL, CAR14 = 0.1 mmol/L,).
Fig. 4
Effect of Pr doping content on the degradation of AR14 (pH = 5, mphotocatalyst = 20 mg, VAR14 = 20 mL, CAR14 = 0.1 mmol/L,).

In order to provide a reasonable explanation for the above phenomenon, XPS spectra of C 1 s was applied and the results are illustrated in Fig. 5. The C1s spectrum in Fig. 5 can be deconvoluted into five different peaks corresponding to the following bonds: C⚌C bond of sp2 carbon, C–C bond of sp3 amorphous carbon, C–OH, C⚌O and O–C⚌O bonds, respectively (Teng et al., 2011). It is interesting to find that three different carbon bonds were formed in CeO2@C with lower Pr doping content while four different carbon bonds were detected in CeO2@C with higher Pr doping content, implying that high Pr doping content is helpful for the formation of carbon bonds in CeO2@C. The possible reason may be that the carbon obtained from the pyrolysis of AR14 is active and the electrons in 2p orbit of carbon is easy to combine with the positively charged oxygen vacancies caused by a high concentration of Pr doping. Therefore, the distance between carbon and lattice oxygen in CeO2 decrease, which will result in abundant C-O bonds in the resulted samples. Due to the structural relaxation of CeO2 caused by the oxygen vacancies (Wang et al., 2012), carbon and hydrogen from the pyrolysis of AR14 is easy to interact with oxygen in CeO2 and form C–O and O–H bonds. Moreover, the improvement of positive charge of carbon in C–O bond will easily interact with other carbons to form lots of C–C and/or C⚌C bonds.

XPS spectra of C 1 s for CeO2@C (a), 15 %Ce-Pr-O@C (b), 25 %Ce-Pr-O@C (c), and 35 %Ce-Pr-O@C (d).
Fig. 5
XPS spectra of C 1 s for CeO2@C (a), 15 %Ce-Pr-O@C (b), 25 %Ce-Pr-O@C (c), and 35 %Ce-Pr-O@C (d).

In order to verify the above inference, photocurrent experiment was carried out and the results are presented in Fig. 6. From Fig. 6, it is obvious that the photocurrent on 25 %Ce-Pr-O@C and 35 %Ce-Pr-O@C is higher than that on CeO2@C, while the photocurrent on 15 %Ce-Pr-O@C is lower than that on CeO2@C, implying more efficient photoelectron-hole separation efficiency for the Ce-Pr-O@C samples with high Pr doping content. However, the photocurrent on 25 %Ce-Pr-O@C is higher than that on 35 %Ce-Pr-O@C, which is not in agreement with the increase of oxygen vacancy concentration with doping Pr. This may be that excessive oxygen vacancies will become the recombination center of electrons and holes (Mittal et al., 2018). The results of Fig. 6 can provide effective evidences for the photocatalytic results of Fig. 4.

Photocurrent-time profiles of CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C.
Fig. 6
Photocurrent-time profiles of CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C.

Besides C–O, C–C, and C⚌C bonds, the metal carbon bonds are also helpful for the improvement of photocurrent. According to Gregson et al., trivalent lanthanide ions are easier to form lanthanide carbon bond than that of tetravalent ones (Gregson et al., 2013). So, we can infer that the photocurrent on Ce-Pr-O@C will be enhanced when the content of trivalent lanthanide ions is increased. How does the content of trivalent lanthanide ions change after Pr doping in CeO2@C? The quantification of trivalent lanthanide ions was carried out by the aid of XPS technique. Fig. 7a shows the XPS spectra of Ce 3d for CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C. The main characteristic peaks of Ce4+ 3d3/2 and Ce4+ 3d5/2 are arose at 897.6 and 915.8 eV, respectively, whereas the peaks located at 881.8 and 900.3 eV are assigned to Ce3+ 3d5/2 and Ce3+ 3d3/2, respectively (Choi et al., 2016). Furthermore, another two peaks located at 883.7 and 888.2 eV can be ascribed to Ce3+ 3d5/2, and the peak at 907.1 eV may be assigned to Ce3+ 3d3/2. From the deconvoluted results of Ce 3d XPS, it can be concluded that the content of Ce3+ in 25 %Ce-Pr-O@C is higher than that of other samples because the ratio of Ce3+ 3d peak area to Ce 3d peak areas of CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, 35 %Ce-Pr-O@C is 38.85 %, 38.67 %, 42.52 %, 39.19 %, respectively. XPS spectra of Pr 3d for 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C are shown in Fig. 7b. According to Pu et al., the peaks at around 953.3 and 932.7 eV can be assigned to Pr4+ and the signals at about 948.8 and 928.3 eV can be associated with Pr3+ (Pu et al., 2007). It can be concluded from Fig. 7b that the content of Pr3+ in 25 %Ce-Pr-O@C is higher than that of 15 %Ce-Pr-O@C and 35 %Ce-Pr-O@C because of the higher peak intensity at about 948.8 and 928.3 eV for 25 %Ce-Pr-O@C. Based on the results of Fig. 7, we can infer that lanthanide carbon bonds can be easily formed in 25 %Ce-Pr-O@C due to the higher content of Ce3+ and Pr3+, resulting in higher photocurrent and thus more efficient photocatalytic efficiency.

XPS spectra of Ce 3d for CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C (a) and of Pr 3d for 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C (b).
Fig. 7
XPS spectra of Ce 3d for CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C (a) and of Pr 3d for 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C (b).

As discussed above and reported by the related research (Silva et al., 2023), increasing of adsorption capacity of organic pollutants over the photocatalyst will improve the photocatalytic efficiency. From our previous report, it is found that the content of hydroxyl groups on the surface of photocatalyst play an important role in the photocatalytic efficiency (Hao et al., 2014a,b). Generally, the content of hydroxyl groups can be reflected by the XPS spectra of O 1 s (Wang et al., 2013). Fig. 8 shows the XPS spectra of O 1 s for CeO2@C, 15 %Ce-Pr-O@C, 25 %Ce-Pr-O@C, and 35 %Ce-Pr-O@C. According to Fan et al., the peak at 528.17–529.5 eV can be assigned to the lattice oxygen and the peak at about 531 eV is attributed to oxygen species in the defects (Fan et al., 2017). From the viewpoint of Venkataswamy et al., the peak at about 533 eV can be associated with the hydroxyl-like groups (Venkataswamy et al., 2014). It can be seen from Fig. 8 that doping Pr is helpful for the formation of hydroxyl groups because no peak at about 533 eV is detected in CeO2@C. In order to investigate the effect of Pr doping on the content of hydroxyl content, the curves of peaks at about 533 eV in Fig. 8(b), (c) and (d) are magnified and the results are presented in Fig. 9. It is clear from Fig. 9 that the peak intensity of 25 %Ce-Pr-O@C is higher than that of 15 %Ce-Pr-O and 35 %Ce-Pr-O@C, implying the highest hydroxyl group content and hence the maximum adsorption capacity for 25 %Ce-Pr-O@C. Furthermore, it can be also concluded from Fig. 1b that the adsorption capacity of AR14 over 25 %Ce-Pr-O@C is the maximum one because of the highest BET surface area. In order to testify the above inference, photocatalysis kinetic was carried out and the results are shown in Fig. 10. From Fig. 10, it can be seen that the adsorption efficiency of 25 %Ce-Pr-O@C is actually higher than that of 15 %Ce-Pr-O and 35 %Ce-Pr-O@C due to the higher removal rate of AR14 for 25 %Ce-Pr-O@C in the dark reaction. Consequently, the photocatalytic efficiency of 25 %Ce-Pr-O@C is higher than that of 15 %Ce-Pr-O and 35 %Ce-Pr-O@C. The photocatalytic efficiency of 25 %Ce-Pr-O@C is also higher than that of corresponding catalysts, such as Gd-doped ZnS QDs/g-C3N4 (Amani-Ghadim et al., 2022) and F-CdTe QD/CoNiAl LDH nanocomposite (Khodam et al., 2022).

XPS spectra of O 1 s for CeO2@C (a), 15 %Ce-Pr-O@C (b), 25 %Ce-Pr-O@C (c), and 35 %Ce-Pr-O@C (d).
Fig. 8
XPS spectra of O 1 s for CeO2@C (a), 15 %Ce-Pr-O@C (b), 25 %Ce-Pr-O@C (c), and 35 %Ce-Pr-O@C (d).
The content of hydroxyl content in different of Ce-Pr-O@C samples.
Fig. 9
The content of hydroxyl content in different of Ce-Pr-O@C samples.
The photodegradation of 0.1 mmol/L AR14 in the presence of different photocatalysts under visible light irradiation (pH = 5, mphotocatalyst = 20 mg, VAR14 = 20 mL).
Fig. 10
The photodegradation of 0.1 mmol/L AR14 in the presence of different photocatalysts under visible light irradiation (pH = 5, mphotocatalyst = 20 mg, VAR14 = 20 mL).

From the result of Fig. S3, it can be seen that the carbon content from a 0.075 mM of AR14 solution is the optimum one for the photocatalytic efficiency of 25 %Ce-Pr-O. From the results of Fig. S4, it can seen that the superoxide radical is the main active specie responsible for the photodegradation of AR14. Based on the above discussion, the possible photocatalytic mechanism for the photocatalytic degradation of AR14 on 25 %Ce-Pr-O was proposed, which is presented in Fig. S5. From Fig. S6, it can be seen that 25 %Ce-Pr-O@C is highly stable after four cycles of photocatalysis.

4

4 Conclusion

Ce-Pr-O@C was synthesized using AR14 as carbon source and effect of Pr doping on the photocatalytic efficiency of Ce-Pr-O@C was investigated in this study. The results indicate that moderate Pr doping is beneficial for the improvement of photocatalytic efficiency of CeO2@C, and that 25 %Ce-Pr-O@C has the best performance among all the Ce-Pr-O@C samples. The reasons are summarized as follows. On the one hand, the adsorption efficiency of AR14 over 25 %Ce-Pr-O@C is higher than that of other Ce-Pr-O@C samples due to higher content of hydroxyl group and BET specific surface area for 25 %Ce-Pr-O@C. On the other hand, a better separation efficiency of photogenerated electrons and holes in 25 %Ce-Pr-O@C due to the fact that the content of carbon bond in 25 %Ce-Pr-O@C is higher than that of other samples.

CRediT authorship contribution statement

Fanfan Zhang: Data curation. Chenxin Mao: Data curation. Yanhong Tu: Formal analysis. Guoju Chang: Data curation. Paolo Aprea: Writing – original draft. Shiyou Hao: Writing – review & editing.

Acknowledgements

The financial support by the National Natural Science Foundation of China (21876158) and Key Science and Technology Projects in Jinhua City of Zhejiang Province (2022-1-077) are gratefully acknowledged.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2024.105874.

Appendix A

Supplementary data

The following are the Supplementary data to this article:

Supplementary Data 1

Supplementary Data 1


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